Selective Extraction of Scandium by a Long Alkyl Chain Carboxylic Acid/Organophosphonic Ester Binary Extractant

The organophosphorus extractant 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) is widely used for solvent extraction of rare earth elements in an acidic leaching solution, but stripping of the loaded metals is difficult because of the high affinity between the metals and the extractant. Adding 100 times the concentration of neodecanoic acid (Versatic 10) to the extractant solution reduces the extraction ability for scandium (Sc) and enables quantitative stripping of the loaded Sc with a mild acidic solution, such as a 1 M mineral acid. The loading capacity is dependent on the PC88A concentration, so PC88A is the main species responsible for Sc extraction in the binary mixture. Nuclear magnetic resonance spectroscopy (NMR) of the organic phases suggests a potential interaction between the mixed extractants through the change in the hydrogen bonding. This causes an antagonistic effect and facilitates efficient Sc stripping from the extractant solution, so this method could be used for scandium recovery in industry.